Typically, mine roof bolting assemblies comprises a threaded roof bolt, which may include one or more sections; a bearing plate; and a nut for holding the bearing plate against the mine roof when the bolt is installed in a bolt hole drilled into the mine roof. Many roof bolting assemblies utilize threaded or non-threaded roof bolts wherein the bolt is driven by the bolt head which has a collar for supporting a bearing plate against the mine roof. The bearing plates are usually made from steel or other strong metal material, are embossed with a dome, bell, cup, donut or other shape, and include a central aperture through which the roof bolt extends. Such bearing plates are shown in U.S. Pat. Nos. 3,090,203 and 4,445,808. Sometimes the bearing plates are provided with reinforcing ribs or sidewalls as shown in U.S. Pat. Nos. 4,112,693 and 4,371,293.
In some mine roof bolting assemblies, flexible, annular washers or gaskets have been placed between the mine roof and the bearing plate for excluding ambient air and moisture from the bolt hole to prevent spalling or crumbling of the sidewalls of the bolt hole. Such sealing devices are shown in U.S. Pat. Nos. 4,183,699 and 4,188,158.
Anti-friction washers have also been used in roof bolting assemblies. They are placed between the bearing plate and the nut or head of a bolt. The purpose of these washers is to relieve friction between the plate and the nut or bolt head, thereby permitting application of a more consistent torque on the roof bolt during installation. The washers also prevent the roof bolt head, nut or other securing means from being pulled through the aperture in the bearing plate. A typical anti-friction washer is flat, approximately 1/8 inch (0.317 cm) thick, square or round with a 2 inch (5.08 cm) side or diameter and hardened to Rockwell C 35-45 range.
Roof bolting assemblies provided with such flat anti-friction reinforcing washers are adequate for normal loading, but are inadequate under heavy loading. For example, a Grade 55, 5/8 inch (1.58 cm) bolt has a load strength in the range of 15,000 to 17,000 pounds. Typically, such a bolt is combined with a 6".times.6".times.3/16" (15.2.times.15.2.times.0.476 cm) bearing plate embossed with a donut or cup shape to produce a bolting assembly with a load capacity of about 20,000 pounds.
With the appearance of stronger primary roof bolts, such as a 3/4 inch (1.91 cm), Grade 75 roof bolt, assemblies having load strengths in excess of 31,000 pounds became possible. In order to achieve these higher assembly load strengths, the thickness of the bearing plate was increased, and usually doubled. The thicker bearing plate was also flattened for easier manufacturing. Then it was coupled with a thin, flat, annular anti-friction washer. However, by making the bearing plate thicker and flatter, the strength advantages from embossing were lost. In addition, substantially more material was required to produce the thicker, flat bearing plate than the earlier embossed plate.
I have found that all of the strength advantages of an embossed bearing plate can be retained by reinforcing it with a washer having a shape that is complementary to the embossed plate, instead of with a flat washer. My reinforcing washer also substantially reduces the manufacturing costs of existing mine roof bolt assemblies. Instead of sacrificing the loading capacity of such assemblies, my invention rather surprisingly increases it.